The present invention relates to a multilayer ceramic capacitor and a method for the manufacture thereof; and, more particularly, to a multilayer ceramic capacitor having good electrical characteristics without a short failure between internal electrodes made of a base metal such as Ni or Cu, and a method for the manufacture thereof.
There is shown in FIG. 1, a cross sectional view of a prior art multilayer ceramic capacitor. As shown, the multilayer ceramic capacitor includes a reoxidized ceramic body 10, a pair of external electrodes 12 installed at two opposite sides of the reoxidized ceramic body 10, a plurality of ceramic dielectric layers 16 stacked on top of another, and a plurality of internal electrodes 14 each of which is provided on top of said each ceramic dielectric layer 16.
The respective internal electrodes 14 and the ceramic dielectric layers 16 are alternately stacked as an integral body. Further, two internal electrodes 14 adjacent to each other are electrically connected to two corresponding external electrodes 12.
The ceramic dielectric layers 16 are made of, e.g., a barium titanate-based dielectric material, and the internal and the external electrodes 14, 12 are made of, e.g., a conductive paste. The conductive paste is obtained by mixing Ni or Cu metal powder as a major constituent with a ceramic powder having an identical composition as the ceramic dielectric layers 16, and sintering the mixture.
An exemplary process for forming the multilayer ceramic capacitor is described below.
First, the barium titanate-based ceramic powder is mixed with an organic binder and water to form a ceramic slurry. Thereafter, ceramic green sheets having a thin sheet shape are formed from the slurry by using, e.g., a doctor blade.
Then, internal electrode patterns made of the conductive paste are printed on the top surfaces of portions of the ceramic green sheets by using a screen print method and are dried, wherein the ceramic green sheets having the internal electrode patterns printed thereon are referred to as xe2x80x9cprinted ceramic green sheetsxe2x80x9d, and the ceramic green sheets having no internal electrode patterns thereon are referred to as xe2x80x9cbare ceramic green sheetsxe2x80x9d. The conductive paste may be prepared by dispersing Ni metal powder into a mixture having an organic binder such as ethyl cellulose resin, unsaturated polyester resin or butyral resin, a solvent such as xcex1-terpineol or toluene, and a vehicle containing a surfactant.
Next, the printed ceramic green sheets are stacked and then the bare ceramic green sheets are stacked against the outermost sheets of the stacked ceramic green sheets to serve as a protecting layer, thereby forming a laminated ceramic body. Subsequently, the laminated ceramic body is pressed in a direction in which the sheets are stacked, and diced into chip-shaped multilayered ceramic bodies.
Thereafter, the chip-shaped multilayered ceramic bodies are loaded into a furnace to undergo a firing process. The multilayered ceramic bodies are calcined in an oxidative atmosphere at a temperature of about 600xc2x0 C. to remove the organic binder contained therein, and then in a non-oxidative atmosphere, e.g., a reductive and/or a neutral atmosphere, are heated at a higher temperature than about 1000xc2x0 C. to sinter the ceramic green sheets and the internal electrode patterns incorporated therein.
After lowering the temperature of the furnace from about 1000xc2x0 C. to about 600xc2x0 C. and changing the non-oxidative atmosphere thereof to the oxidative atmosphere, the sintered ceramic bodies are reoxidized to form reoxidazed ceramic bodies. The reoxidization process is performed so as to improve the insulation of the ceramic layers which may have been deteriorated during the sintering process carried out in the non-oxidative atmosphere.
Finally, after the paste for the external electrodes is applied on two opposite sides of each of the reoxidized ceramic bodies, annealing is carried out on the reoxidized ceramic bodies to install the external electrodes thereon, thereby forming the multilayer ceramic capacitor.
In such a method for manufacturing the multilayer ceramic capacitor, while the reoxidization process is carried out in the sintered ceramic body, sufficient oxygen is not fed into a central portion(K) surrounded by a broken line(see FIG. 1) which may in turn hamper the ceramic layers within the central portion of the sintered ceramic body from being reoxidized sufficiently. This may deteriorate an insulation resistance(xe2x80x9cIRxe2x80x9d) of the ceramic layers within the central portion, to thereby cause an early failure of the multilayer ceramic capacitor.
In order to solve the above problem, attempts have been made to add an extra amount of the acceptor; however, such would lower the dielectric constant or the life time of the capacitor.
It is, therefore, an object of the present invention to provide a multilayer ceramic capacitor having a central portion capable of being easily oxidized during an oxidization process and a method for the manufacture thereof.
In accordance with one aspect of the present invention, there is provided a multilayer ceramic capacitor comprising:
a multilayer ceramic body including alternately stacked ceramic layers and internal electrodes, the ceramic layers having an acceptor therein, the acceptor representing one or more metallic elements for facilitating a reoxidation process of the multilayer ceramic body; and
a pair of external electrodes installed at two opposite sides of the multilayer ceramic body,
wherein the acceptor concentration in the ceramic layers stacked at a central portion of the multilayer ceramic body is higher than that in the remaining ceramic layers of the multilayer ceramic body.
In accordance with another aspect of the present invention, there is provided a method for manufacturing a multilayer ceramic capacitor comprising the steps of:
forming a laminated ceramic body, the laminated ceramic body including alternately stacked ceramic green sheets and internal electrodes;
sintering the laminated ceramic body in a non-oxidative atmosphere to form a sintered ceramic body; and
reoxidizing the sintered ceramic body in an oxidative atmosphere to form a reoxidized ceramic body,
wherein the ceramic green sheets have an acceptor therein, the acceptor representing one or more metallic elements for facilitating the reoxidizing step, and the acceptor concentration in the ceramic green sheets at a central portion of the laminated ceramic body is greater than that in the remaining ceramic green sheets of the laminated ceramic body.
In accordance with still another aspect of the present invention, there is provided a method for manufacturing a multilayer ceramic capacitor comprising the steps of:
forming a laminated ceramic body, the laminated ceramic body including alternately stacked ceramic green sheets and internal electrodes;
sintering the laminated ceramic body in a non-oxidative atmosphere to form a sintered ceramic body; and
reoxidizing the sintered ceramic body in an oxidative atmosphere to form a reoxidized ceramic body,
wherein the internal electrodes have an acceptor therein, the acceptor representing one or more metallic elements for facilitating the reoxidizing step, and the acceptor concentration in the internal electrodes at a central portion of the laminated ceramic body is greater than that in the remaining internal electrodes of the laminated ceramic body.